There have been many display devices which utilizing a refractive anisotropy and a dielectric anisotropy, characteristics of liquid crystalline compounds. Those display devices are used widely in watches, electronic computers, word processsors, television sets etc., demands of which have been increased year by year. Liquid crystal phase is positioned between solid phase and liquid phase, and liquid crystal phase is classified roughly into nematic phase, smectic phase, and cholesteric phase, and among them, display devicesutilizing nematic phase are most widely used.
Furthermore, as display methods applied for liquid crystal displays, there are present TN (twist nematic) type, DS (dinamic scattering) type, guest-host type and DAP type etc. concerning to their electric optical effects. In particular, colorization of liquid crystal display has been recently more progressed, wherein the main currents are a thin film transistor (TFT) method and a super twist nematic (STN) method concerning to TN type, and these display devices are mass-produced.
Although a number of liquid crystalline compounds including those in study are known, there is not present any substance which is enclosed and used in display devices as a single liquid crystalline substance. The reasons thereof is that there has not been found any substance to satisfy the following conditions as a single substance: namely, that liquid crystalline compounds to be expected as materials of display devices are desired to show a liquid crystal phase within a wide temperature range as possible in nature centering around the room temperature, which is most often used for display devices, that the compounds should be sufficiently stable against environmental factors to be used, and that the compounds should have physical properties sufficient for driving display devices.
Thus, compositions having such characteristics are prepared and used practically by mixing several liquid crystalline compounds or non-liquid crystalline compounds. These liquid crystal compositions are required to be stable against moisture, light, heat, and air etc. which are generally present in the used environment. In addition, stabilities against electric field and electromagnetic radiation are necessary and also the liquid crystalline compounds mixed are required to be chemically stable each other in the used environment.
Furthermore, liquid crystal compositions are necessary to have suitable values of physical properties such as a refractive anisotropy value (.DELTA.n), a dielectric anisotropy value (.DELTA..epsilon.), a viscosity (.eta.), a conductivity and an elastic constant ratio K.sub.33 /K.sub.11 (K.sub.33 : bend constant, K.sub.11 : splay elastic constant) etc. according to display methods and device forms. Furthermore, it is important that each components in liquid crystal compositions have good solubilities mutually.
Among these physical property values, particularly a wide liquid crystal phase temperature range, a low viscosity and a high elastic constant ratio K.sub.33 /K.sub.11 are required for liquid crystal compounds to be used in a STN type display method. Recently, environments in which display devices being used are divesified so that materials having a wide liquid crystal phase temperature range are necessary from requirements caused by those environments, and a low viscosity is a necessary and indispensable characteristic for attaining a high speed response, and a high elastic constant ratio K.sub.33 /K.sub.11 makes a change in a transmittance sharply in vicinity of a threshold voltage and also makes display devices with a high contrast possible.
In general, compounds having (an) alkenyl group(s) are known to show low viscosities. However, when comparing compounds having (an) alkyl group(s) and compounds having (an) alkenyl group (s), compounds having (an) alkenyl group (s) have such tendencies that smectic properties are decreased but liquid crystal phase temperature ranges are also decreased.
Absolutely, from comparison of compounds (a-1) to (c-2) described in V. Vill, Landort-Velunstein, in bicyclohexyl derivatives (a-1) and (a-2), the clearing point of (a-2) having (an) alkenyl group(s) is about 1.degree. C. higher than (a-1) having (an) alkyl group(s), whereas the melting point of the former is about 23.degree. C. higher than the latter. Therefore, the liquid crystal phase temperature range is decreased by about 22.degree. C. The increasing tendencies of the clearing point and theme lting point are found by comparison of cyclohexylphenyl derivatives (b-1) and (b-2), wherein the liquid crystal phase temperature range is decreased by about 10.degree. C.
Furthermore, in biphenyl derivatives (c-1) and (c-2), liquid crystal phase temperature ranges are decreased by about 37.degree. C. owing to decrease in the clearing point and increase in the melting point. In both cases, the liquid crystal phase temperature ranges are eminently decreased by substituting an alkyl group with an alkenyl group.
Furthermore, as to compounds showing a high elastic constant ratio K.sub.33 /K.sub.11, there have been already known compounds having (an) alkenyl group(s) in side chains such as compound (d) described in Toku-Ko-Hei 4-30382 or compound (e) described in Toku-Ko-Hei 7-2653. These compounds have however poor appearances of liquid crystal phases, and temperature ranges thereof are very narrow even if appearances are obtained. Thus, in the case that they are used as components of liquid crystal compositions, deposition of crystals or appearance of smectic phase may be confirmed at a lower temperature range so that solubility at a lower temperature cannot be said good. ##STR2##